This problem is from the Korean tsumego connection 天龍図, known in Japan as Tenryūzu. The name might be translated into English as ‘Sky Dragon Diagrams’. Most of the problems in the collection are notoriously difficult, making it good practice material for aspiring professional players.
For this Sunday Problem, I tried to pick a relatively easier problem from the collection.
Black to play.
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The first move is difficult. My top 3 choices were all wrong, with option #4 I found the answer.
I also like the ‘honesty’ of this problem: m18–m19 very obviously creates a ko for White’s life, so assuming that the problem is difficult, there has to be an unconditional kill hiding somewhere.
Solution added.